Process for the production of metal nitrides



April 19, 1949. 2,467,647

PnocEss Fon 'um PRODUCTION oF METAL NI'rnIDss P.. P. ALEXANDER Filed June 7, 1945 /v/rfraf/vy Han/v6. EME:

M* w Ml V INVENTQR uw P. uom/V058 BY c - ATTORNEY:

Patented Apr. 19, 19u49 PROCESS FOR THE PRODUCTION OF METAL NITRIDES Peter P. Alexander, Beverly, Mass., assigner to Metal Hydrides Incorporated, Beverly, Mass., a corporation of Massachusetts Application June 7, 1945, ASerial No. 598,025

This invention relates to the production of metal nitrides and has for its object certain improvements in the method of producing metal nitrides. The invention relates more particularly to improvements in the method of producing nitrides of the alkaline earth metals, such as calcium nitride.

Little progress has been made in the production of metal nitrides, particularly the alkaline earth metal nitrides, in large quantities on a commercial scale by reacting the metals to be nitrided with nitrogen gas at an elevated temperature. Prior investigators appear to have operated under the belief that the nitriding reaction is carried on more eiectively at a temperature above that at which fusion of the alkaline earth metal occurs because at a temperature below that at which fusion' occurs, the nitriding reaction takes place only at the surface of the metal to be nitrided.V i

Considerable investigation has confirmed my conclusion that the customary methods of producing metal nitrides, such as those just referred to, are inherently defective because the metals to be nitrided are fused and that greatly improved results may be obtained by conducting the nitriding reaction Without fusion of the metal to be nitrided, provided the method employed is suitable to the character of the metal employed. This permits the ready production of large quantities of metal nitrides.

overheating the metal to be nitrided'is objectionable for a number of reasons: Incomplete conversion of the metal to its nitride results from fusion of the metal. Metal nitride made by the fusion method is easily contaminated by the metal retort in which it is made. Gaseous impurities, when present in the nitrogen. gas employed, tend to reach with the metal to be nitrided. There is also a tendency for the metal nitride produced to dissolve in the fused metal. For' example, in the .production of calcium nitride, the calcium nitride tends to dissolve in the fused calcium, thereby greatly impairing the efficiency of the nitriding reaction. In addition, it is difficult to remove the fused metal nitride from the reaction vessel or retort, thus requiring a great deal more labor which adds considerably to the cost of conducting the operation. It willv be clear that the customary methods of producing metal nitrides by reacting metals with nitrogen leave much to be desired.

i To initiate the nitriding reaction, the metal to be nitrided is heated to an elevated temperature in the presence of nitrogen gas. This is Vdone 9 Claims. (Cl. 23-191) with applied heat, for example, by heating the retort externally in which the nitriding'reaction is conducted. As is well known, the reaction begins at a temperature below the fusion point of the metal. When the metal and nitrogen combine to form metal nitride, vexothermic heat is generated. In the prior methods referred to above, it is the applied heat alone or the applied heat plus the generated heat which fuses the metal to be nitrided. At the nitriding temperature, the metal to be nitrided has a pronounced affinity for nitrogen. This is particularly true of the more reactive metals, such as the alkaline earth metals and the alkali metals. For a given metal, the rate at which the exothermic heat is generated depends for the most part on the amount of surface exposed by the metal to the nitrogen.

If a charge of the metal to be nitrided is in finely divided form, small pieces, a relatively large or enormous amount of surface area, compared with the weight or volume of the metal, is exposed to the nitrogen; and the rate of generation of exothermic heat is extremely rapid, particularly with the more reactive metals. The smaller the pieces, the less exothermic heat they can absorb before interior portions thereof are heated to the point at which nitriding can take place. Unless special precautions are taken, the amount of heat generated is sufiicient promptly to fuse or melt the metal and sometimes the metal nitride, or to dissociate the metal nitride. The rise in temperature may be so instantaneous as to cause the reaction to take place with explosive violence. This is particularly true of the alkali metals and, to a lesser extent, of the alkaline earth metals.

If, on the other hand, a charge of the metal to be nitrided is in a large piece or pieces, a relatively small amount of surface area, compared with the Weight or volume of the metal, is exposed to the nitrogen; and the rate of generation of exothermic heat is rather slow, compared with the preceding example, even in the case of the alkaline earthmetals. The larger the piece or pieces, the more exothermic heat they vcan absorb before interior portions thereof are heated to the point at which nitriding can take place. Nevertheless, unless special precautions are taken, the amount of exothermic heat generated may be suflicient to fuse or melt the metal, as well as the metal nitride, or even to dissociate the metal nitride.

In other words, for a given weight of metal to be nitrided, the smaller the pieces of metal amaca the larger the amount of surface area exposed to the nitrogen by the pieces, and hence the faster the rate of generation of exothermic heat;

f and the larger the pieces of metal, the smaller the amount of surface area exposed to the nitrogen by the pieces, and hence the slower the rate of generation of exothermic heat. To avoid fusion or melting of the metal, the method employed is therefore adapted particularly to the size of the pieces of metal. In the case of metallic calcium, for example, it is commercially available in a number of forms, such as relatively large ingots or slabs obtained in the, customary electrolytic process of production, fairly large rods and the relatively small crystals or pieces obtained in the customary pyrometallurgical process of production. The method `of nitriding is varied according to the size of the pieces to prevent fusion of the calcium.

In all cases, however, after the nitriding reaction is initiated by the applied heat, it is aimed to balance the amount of heat generated in the reaction zone with the amount of heat dissipated from the reaction zone so as to maintain the re-` action zone at a temperature high enough to permit the nitriding reaction to go to completion and at the same time low enough to prevent fusion of the metal. This may now be done effectively in various ways. The amount of applied heat is limited to that necessary to start the nitriding reaction and is insufficient to fuse the metal. IIhe nitriding reaction is thereafter conducted under operating conditions, as stated, which prevent fusion of the metal.

In accordance with the invention, a charge of the metal to be nitrided is heated with applied heat in the absence of air and in the presence of a limited amount of nitrogen gas diluted with an inert gas in a reaction zone to a temperature sufflciently high to cause the nitrogen to begin to combine with the metal to form the desired metal nitride but insuiiiciently high lto cause the metal to fuse. Additional amounts of nitrogen gas are gradually admitted into the reaction zone as the conversion of the metal to its nitride progresses so that the nitrogen gas diffuses through the metal nitride already formed toward the interior of the body of metal. Generated heat is dissipated from the reaction zone. The temperature in the reaction zone is maintained below that at which the metal fuses but at which the nitriding reaction takes place as the nitriding reaction goes to completion by regulating the rate at which` nitrogen gas is admitted to the reaction zone and by regulating the rate at which generated heat is dissipated from the reaction zone. The resulting metal nitride is cooled, after which it is removed from the reaction zone in its unfused condition.

The charge of metal to be nitrided is advantageously confined within the reaction zone of a retort capable of being heated externally. The retort is provided with a removable cover adapted to seal the retort against ingress of outside air.

Through a series of suitable connections the interior of the retort is connectible with a source of vacuum and nitrogen gas; and, if desired, inert gas. r

In a presently preferred practice, the applied heat is used only in amount to give the nitriding reaction a good start and thereafter heat generated solely by the exothermic reaction is used only in amount to carry the nitriding reaction to completion without fusion'of the metal, the excess generated heat being dissipated from the reaction zone. To this end, the retort may be externally or otherwise heated to provide the applied heat. After the nitriding reaction begins, further use of applied heat is therefore terminated; although it may, of course, be used again if the temperature of the reaction zone should fall below that at which the nitriding reaction takes place.

Various procedures may be followed to balance the amount of heat generated in the reaction zone with the amount of heat dissipated from the reaction zone to maintain the reaction zone at the nitriding temperature but below the fusion temperature of the metal while more and more and finally all of the metal is converted to its nitride: In the case particularly of relatively large pieces of metal, it is highly desirable to maintain the desired nitriding temperature by dissipating generated heat from the reaction zone at the necessary rate. This may be eiTected, for example, by using a retort having the proper amount of radiating surface to dissipate the excess generated heat from a charge of metal of given weight. The retort may, in turn, be permitted to remain in a position where the necessary dissipation of heat takes place automatically, for example in the open air but preferably in a conned space to prevent stray and intermittent currents of air from 'affecting unduly the rate of heat transfer at the Wall of the retort, the aim being to obtain a steady and regular rate of heat transfer under substantially the same nitriding conditions. If the temperature within the reaction zone should rise unduly, the rate of heat dissipation may be speeded, and hence the rate of formation of the metal nitride slowed; for example, by blowing cooling air against the retort. If, on the other hand, the temperature within the reaction zone should fall unduly, the rate of heat dissipation may beslowed or reversed, and hence the rate of formation of the metal nitride speeded, by the use of more applied heat; for example, by passing heating gases temporarily against the retort.

The temperature within the reaction zone, containing a large amount of metal, of relatively large pieces compared with the size of the retort, may be maintained within the desired range entirely by regulating the amount of nitrogen gas admitted to the retort, with a small amount of unavoidable heat dissipation therefrom. Small amounts of nitrogen admitted in this manner tend to be consumed quickly by the metal, particularly an alkaline earth metal. This is a slow operation, requiring extremely accuratev controls, which may get out of working order; and is apt to create a vacuum in the retort, if the only gas in the reaction zone is nitrogen, which may cause it to spring a leak and thus permit outside air and moisture to seep into the retort, where they may react with the met-al. This contingency may be avoided by admitting enough inert gas to the retort to keep the reaction zone and its charge under positive pressure, that is, above atmospheric pressure. 'I'o avoid the creation of a vacuum in the retort by the rapid consumption of nitrogen in the former case, and to avoid the use of inert gas in the latter case, I prefer to admit enough nitrogen gas to the retort to keep the reaction zone and its charge continuously under slight positive pressure and then to dissipate the excess generated heat from the retort to maintain the desired temperature range in the reaction zone. The end desired may be obtained in various Ways.

In the case particularly of relatively small pieces of metal, and especially the more reactive metals, like those in the alkaline earth metal amount of surface area exposed by the small pieces of metal, nitrogen gas admitted to the reaction zone is very quickly consumed. The greater the amount of nitrogen thus reacting with the metal, the greater is the amount of heat generated. If, for example, a large amount of metal, of relatively small pieces, compared with the size of the retort, is placed within the reaction zone and inert gas is not admitted thereto, the desired temperature range can be maintained only by regulating the amounts of nitrogen gas thus admitted to the retort. This is again a slow operation, requiring extremely accurate controls, which may cause it to spring a leak and thus permit outside air and moisture' to seep into the retort where they mayreaot with the metal.

A better procedure in such a case is carefully to regulate both theamount of nitrogen gas admitted to the reaction zone and the rate of heat dissipated from the reaction zone. The rate'of heat generation may be slowed by slowing the rate of nitrogen gas admitted to the reaction zone and the rate of heat dissipation may be speeded by cooling the reaction zone the necessary amount. Lowering the temperature of the reaction zone operates to slow the rate of formation of the metal nitride. In other Words, in this practice, reliance is placed on two forms of regulation, that of the amount of nitrogen gas admitted to the reaction zone and that of the amount of generated heat dissipated from the reaction zone per nit of time. Even in such a case, the nitrogen gas must be admitted in such small amounts that it is quickly consumed in the nitriding reaction, thus creating a vacuum l in the reaction zone and leading to the danger of a leak.

A still better procedure, when nitriding relatively small pieces of metal, is to fill the reaction zone with a suitable inert gas, such as helium or argon, preferably in amount to place and maintain the reaction zone and charge under substantial positive pressure. Regulated small amounts of nitrogen gas may then be admitted to the reaction zone and be consumed in the nitriding reaction. 'Ihe inert gas functions to dilute the nitrogen gas and to prevent the reaction zone from being placed under vacuum on loss of nitrogen in the nitriding reaction. It is particularly advantageousto combine regulation of the rate of admission of nitrogen gas to the reaction zone with regulation of the rate of dissipation of excess generated heat from the reaction zone to maintain the desired nitriding temperature range.

The rate of consumption of nitrogen gas by a given metal of given Weight appears to be directly proportional to the amount of surface area exposed by the metal to the nitrogen. When nitriding pieces of metal of intermediate size, the method employed is adapted to ythat size. For practical purposes, the line of demarcation is whether the operation creates a vacuum in the reaction zone. In general, it is preferred to nitride pieces of the metal sui'liciently large to require no more than a regulation of the rate of nitrogen gas admitted to the reaction zone plus a regulation of the rate of heat dissipation from the reaction zone to maintain the desired temperature range without creating a vacuum in the reaction zone. If this cannot be done, because the pieces of metal are too small, the next lbest procedure is to admit enough inert gas to the reaction zone to place it under positive pressure and then to use both forms of regulation.

These and other features of the invention will be better understood by referring to the accompanying drawing, taken in conjunction with the following description, in which- Fig. 1 ls a diagrammatic representation of an apparatus employable in the practice of the method of the invention, being a side elevation mostly in cross-section, showing the use of an inner and outer retort, the inner retort containing 'a charge of relatively large calcium rods packed therein;

Fig. 2 is a top or plan view of the inner retort, showing the top or upper end portions of the calcium rods;

Fig. 3 is a similar top or plan view of the inner retort, showing the top or upper end portions of a charge of relatively large calcium ingots packed therein; and

Fig. 4 is a side elevation in section of the inner retort, showing a charge of relatively small pieces of calcium therein.

The apparatus shown comprises an outer retort I0 supported within a heating furnace I I, the retort being held in position by means of a plurality of supports I2 spaced under and around a cir- 1 cumferential flange of the retort and resting on th top of the heating furnace. The retort is made preferably of heat-resistant steel. It is provided with a removable cover I3 having attached thereto a vertical pipe I4 with valved branches I5, I6 and Il connectible with a source of vacuump nitrogen gas and inert gas, such as helium or argon, respectively. A gasket I8 is disposed between the cover and the flanged top portion of the outer retort,- the three elements forming a non-leakable connection by means of a plurality of spaced bolts I9. A removable inner retort 20 having an open top rests within the outer retort.

The heating furnace consists essentially of a box-like chamber 2| having a refractory bottom 22, side and end Walls 23 and a top 24 with an opening of a size adapted to receive the outer retort. An expanding opening 25 is provided at or near a lower corner of one of the side walls ci the chamber for the introduction of heating ga'es into the chamber. A flue opening 26 extends through another wall, preferably at a higher level, so that heating gases passed into the chamber through opening 25 tend to pass around and in contact with the outer retort before leaving the chamber through the iiue opening.

The apparatus may be used as follows: Referring first to the practice illustrated by Figs. 1 and 2, a charge of calcium rods 30, for example, is clo'ely packed in inner retort 20. Round calcium rods are available having a diameter of about 11/4. A plurality of these rods having a length of about 2 feet may be placed on end and tightly packed against one another to ll the lower portion of the inner retort. Since the rods are round, the vertical spaces between adjacent rods provide passageways for nitrogen gas. The inner retort charged in this manner is placed within outer retort I0. Cover I3 is then securely bolted to the outer retort. In a presently preferred practice, the charge of calcium rods .weighs a predetermined amount and the outer retort is provided with a'predeterminedamount of radiating surface to facilitate balancing of the heat generated within the retort and that dissipated from the retort. Heating gases are passed through opening 2 5 into heating chamber 2| where they circulate around and in contact with outer retort I0. After giving up their heat, the spent gases are exhausted through exit 26. While the outer retort and hence inner retort 20 and the charge of calcium rods are being heated in this manner (it is advantageous to place the retorts under vacuum. To this end, the valve in branch I5 is connected with a source of vacuum, not shown. vAlthough the retorts may be evacuated at normal temperatures, removal of the undesired air and moisture is facilitated by elevating the temperature of the reaction zone. Before the nitriding temperature is reached, the valve in branch I5 is closed.

The valve in branch I6, which connects a source of nitrogen gas, not shown, is then opened to admit nitrogen gas to the retorts. The valve is preferably a pressure valve so that a predetermined pressure of nitrogen gas may be maintained continuously in the retort. This pressure is preferably above atmospheric pressure, the object being to assure a constant supply of nitrol gen gas in the retorts to replace that consumed in the nitriding reaction; to assure a continuous nitriding reaction so that exothermic heat is continuously generated; and to prevent a vacuum from being created in the retorts so that air and moisture cannot enter through a leak. While, as stated above, inert gas may also be admitted to the reaction zone when nitriding relatively large pieces of calcium, such as those described, its use is not necessary. If desired, however,` the valve in branch I'I, which is connected to a source of inert gas, not shown, may be opened to admit inert gas. f

Suilicient heating gases are passed into the heating chamber at a temperature sufficiently high to heat the charge of Acalcium rods to a temperature sufficiently high to give the nitriding reaction a good start, but below the temperature of fusion of the calcium. Further introduction of heating gases into chamber 2| is then discontinued. If desired, thev` retort assembly may be left suspended in the heating chamber, or it may be transferred preferably to a confined space shielded from stray currents of air, provision however being made to continue the flow of nitrogen gas into the retorts. In either case, the object is to dissipate the excess heat generated within the retorts by the exothermic nitriding reaction, the excess being the generated heat not required to maintain the charge at the nitriding temperature and below the fusion temperature of the calcium. In other words, the dissipation of heat is so regulated that the heat of formation is just suflicient to maintain the desired nitriding temperature and yet not heat the calcium to its temperature of fusion.

The instant metallic calcium in the bottom of inner retort II) is brought to the nitriding temperature, the calcium begins to absorb or react with nitrogen supplied to the retort. The absorption of nitrogen by one or more of the calcium rods or part of the rods results in the elevation of the temperature in that part of the retort due to the heat of reaction. The adjacent calcium rods then rise in temperature until they. too, begin to react with nitrogen, with the generation of more heat. The application of applied heat, as pointed out above, is discontinued and the nitriding reaction is permitted to proceed by itself, the necessary heat therefor being supplied by the generated heat. With a properly de:igned retort having a sufllcient area of radiation, the heat generated inside of the retort will be sufl'lcient tomaintain the reaction zone at an optimum nitriding temperature but below that at which the calcium fuses, the excess heat being dissipated from the charge and retorts. Once the equilibrium or balance between the heat generated in the retort and the heat dissipated by the external surface of the outer retort is reached, the nitriding reaction proceeds automatically and without any additional labor or supervision.

The nitrogen gas is absorbed gradually since it has to diffuse through a solid layer of previously formed calcium nitride on the relatively large pieces of calcium. This eliminates the possibility of an inrush of nitrogen gas and heating of the calcium to its melting point. The time required to complete the diffusion of .nitrogen gas through the solid rods of calcium and to complete the conversion of thev calcium to calcium nitride depends of course on the amount of calcium in the charge and the size of the retorts.

When allor the calcium is converted to calciuml nitride, there is no more generation of exothermic heat. 'I'he retorts and the calcium nitride, therefore, cool to room temperature, which is an indication" that the nitriding reaction is completed. 'I'he valve in branch 4I6 'is closed, cover I3 is removed, inner retort 20 is pulled out of outer retort Ill, and the freshly prepared calcium nitride is" dumped out of the inner retort. Since no fusion takes place, the rods of calcium are converted into rods of calcium nitride.

Referring next yto Fig.f3, inner retort- 20 is shownsimilarly packed with a charge of relatively large ingots of calcium 3l, being rectangular and somewhat larger in cross-section than the particular calcium rods just'described. Such ingots are available and also have the advantage of being regular in shape, which permits them to be handled with a minimum amount of labor. The practice followed in converting the calcium ingots vto calcium nitride ingots is the same as that described in converting the calcium rods to calcium nitride rods. The amount of applied heat necessary to start the nitriding reaction is small. Starting the nitriding reaction may be compared to lighting a wood-pile with a match. After the nre is started, wood in the pile burns progressively from piece to piece as the necessary heat of combustion spreads from piece to piece, until the burning of the entire woodpile is completed. In similar fashion, the charge of calcium nitrides from piece to piece as the necessary heat of nitriding spreads from piece to piece, until the nitriding of the entire charge is completed.

Referring finally to Fig. 4, inner retort 20 is shown loaded with a charge of relatively small y pieces of calcium 32 of more or less indiscriminate size, except that fines are preferably screened therefrom. So far as the preliminaries are concerned, the practice followed is like that in amounts. The nitriding reaction tends to take place almost spontaneously in the presence of much nitrogen, therefore generating an excessive amount of ,exothermic heat. While a retort oi' minimum amount of radiating surface can be employed, provided the nitrogen gas is not fed to the retort fast enugh to cause the nitriding reaction to generate an excessive amount of heat, it is preferred not only to regulate carefully the amounts of nitrogen gas admitted to the retorts but to hasten the rate of heat dissipation from the reaction zone. This may be done, for example, by using a relatively large retort which affords a large amount of radiating surface, or by employing special cooling means to facilitate heat transfer from the reaction zone. Thus, cooling air may be passed in contact with the outer retort, or other cooling facilities may be employed. The smaller the pieces of calcium, the more dimcult it is to control the rate of formation of the calcium nitride. Moreover, the relatively small amounts of nitrogen gas that may be admitted to the retorts are ravenously consumed by the small pieces of calcium, so that in the absence of enough inert gas a vacuum is promptly created in the retorts. While the nitriding reaction may be conducted under vacuum, as pointed out previously if a leak occurs, undesirable air and moisture may seep into the retorts.

If the amount of heat generated in the reaction zone cannot readily be balanced with the amount of heat dissipated from the reaction zone to maintainr the reaction zone at a temperature high enough to permit the nitriding reaction to go to completion and at a temperature low enough to prevent fusion of the calcium until the calcium is converted to its nitride, and if the consumption of nitrogen gas creates a vacuum in the retorts, it is advantageous to open thevalve in branch i1 and admit a suitable amount of inert gas, preferably enough to place the reaction zone under substantial positive pressure. Regulated amounts of nitrogen gas may then be passed into the retorts. In addition, the amount of heat dissipated from the retorts is advantageously regulated. With such controls, the desired temperature conditions may be maintained in the reaction zone.

At atmospheric pressure, calcium begins to nitride at temperatures far below its melting temperature, say in the neighborhood of 300 C.; and it melts at about 810 C. It is therefore desired to conduct the nitriding step between those two temperatures when operating at atmospheric pressure. The nitriding and melting or fusion temperatures tend to vary with pressure.

A good temperature range in which to operate under normal conditions is about 300 C. to somewhat below 800 C., say 760 C., and toward the end of the nitriding reaction, particularly when nitriding the larger pieces of calcium, the temperature of the reaction zone is raised advantageously for a short period to about 900 C., below the melting point of the calcium nitride, to facilitate diffusion of the nitrogen gas through the outer layers of calcium nitride on the pieces of calcium to and absorption thereof by partially nitrided and un-nitrided calcium in their interior portions. In other Words, the eiiiciency of the nitriding reaction is thus increased.

While the above examples deal with the conversion of metallic calcium to calcium nitride, it will be clear that the practice may be followed with other nitride-forming metals, particularly 10 the more reactive metals, such as the alkaline earth metals, including barium and strontium, and the alkali metals. It will also be clear that the practice illustrated in the examples may 'be suitably modied and still fall within the limits of the invention. Metal nitrides of high purity.'

may be readily made in accordance with -the vention herein disclosed.

I claim:

1: In the method of producing metal nitrides, the improvement which comprises placing a, charge of the nitride forming metal in a closed reaction zone, evacuating the reaction zone to remove air and mois/ture therefrom, heating said metal to maintain it above its minimum nitride forming temperature but below its fusion temperature, admitting a limited amount of nitrogen to the reaction zone to initiate the nitriding reaction and limi-t the exothermic heat generated by the reaction to an amount insufficient to raise,

the tem-perature of the reaction zone to said fusion temperature, gradually admitting further amounts of nitrogen to the reaction zone to continue the nitriding reaction, and regulating the rate of formation of lthe metal nitride and hence the rate of generation of exothermic heat by controlling the rate iat which said further -amounts of nitrogen are admitted and thereby maintain the temperature of the reaction zone below said fusion temperature.

2. In the method of producing metal nitrides, the 4improvement which comprises placing a charge of the nitride forming metal in a closed reaction zone, evacuating the reaction Zone to remove air and moisture therefrom, heating said metal to maintain it above its minimum nitride forming temperature but below its fusion temperature, admitting a limited amount of nitrogen to the reaction zon-e to initiate the nitriding reas-tion and limit the exothermic heat generated by the reaction to an amount insuicient to raise the temperature of the reaction zone to said fusi-on temperature, admitting an inert gas to the reaction zone to dilute the nitrogen therein, gradually admitting further amounts of nitrogen Ito the reaction zone to continue the nitriding reaction, and regulating the rate of formation of Ithe metal nitride and hence the rate of generation of exothermic heat by controlling the rate at which said further amounts of nitrogen are -admitted and thereby maintain the :tem-perature of the reaction zone below said fusion temperature.

3. In the method of producing metal nitrides, the improvement which comprises placing a Icharge of lthe nitride forming metal in a closed reaction zone, evacuating the reaction zone to remove air and moisture therefrom, heating said metal to maintain it above its minimum nitride forming temperature but below i-ts fusion temperature, admitting a limited amount of nitrogen :to the reaction zone to initiate the nitriding reaction and limit the exothermic heat generated by the reaction to an amount insuiicient to raise the temperature of the reaction zone to said fusion temperature, admitting an inert gas to the reaction zone to di-lute the nitrogen therein and to maintain the reaction zone under a presure not less than about atmospheric pressure, gradually admitting further amounts of nitrogen to the reaction zone to continue the nitriding reaction, and regulating the rate of formation of the metal nitride and hence the rate of gen-- eration of exothermic heat by controlling -the rate at which said further amounts of nitrogen 11 are admitted and thereby maintain the tempera- 'ture of the reaction zone below said 'fusion temcharge of small pieces of the nitride forming metal in a closed reaction zone, evacuating the reaction zone to remove air and moisture there-A from, heating said metal to maintain it above its minimum Initride forming temperature but below its fusion temperature, -admitting a limited amount of nitrogen to the reaction zone to 'initiate the nitriding reaction and limit the exothermic heat generated by the reaction to an amount insufficient to raise the temperature of the reaction zone to said fusion temperature, gradually admitting further amounts of nitrogen to the reaction zone to continue the ni-triding reaction, and regulating the rate of formation of the metal nitride and 4hence the rate of generation of exothermic heat by controlling the rate at which said further amounts of nitrogen are admitted and thereby maint-ain vthe temperature of the reaction zone below said fusion temperature.

5. In a method of producing metal nitrides, the improvement which comprises placing a charge of small pieces of the nitride forming metal in a l closed reaction zone, evacuating the reaction zone to remove air and moisture therefrom, heating said metal to maintain it above its minimum nitride forming temperature but below its fusion` temperature, admitting a limited amount of nitrogen to the reaction zone to initiate the nitriding reaction and limit the exothermic heat generated by the reaction to an amount insuillcient to raise the temperature of the reaction zone to said fusion temperature, admitting an inert gas to the reaction zone to dilute the nitrogentherein, gradu ally admitting further amounts of nitrogen to the reaction zone to continue the nitriding reac'- tion, and regulating the rate of formation of the metal nitride and hence the rate of generation of exothermic heat by controlling the rate at which said further amounts of nitrogen are admitted and thereby maintain the temperature of the reaction zone below said fusion temperature.

6. In the method of producing metal nitrides, the improvement which comprises placing a charge of small pieces of the nitride formingl metal in a closed reaction zone, evacuating the reaction zone to remove air and moisture therefrom, heating said metal to maintain it above its minimum nitride forming temperature but below its fusion temperature, admitting a limited amount of nitrogen to the reaction zone to initiate the nitriding reaction and limit the exothermic heat generated by the reaction to an amount insuicient to raise the temperature of the reaction zone to ysaid fusion temperature, admitting an inert gas to the reaction zone to dilute the nitrogen therein and to maintain the reaction zone under a pressure not less than about atmospheric pressure, gradually admitting further amounts of nitrogen to the reaction zone to continue the nitriding reaction, and regulating the rate oi' formation of the metal nitride and hence the rate of generation of exothermic heat by controlling the rate at which said further amounts of nitrogen are admitted and thereby maintain the 12 temperature of the reaction zone below said fusion temperature.

7. In the method of producing calcium nitride, the improvement which comprises placing a charge of calcium in a closed reaction zone, evacuating the reaction zone to remove air and moisture therefrom, heating the calcium to maintain it at a temperature between 300 C. and 800 C., admitting a limited amount of nitrogen to the reaction zone to initiate the nitriding reaction and limit the exothermic heat generated by the reaction to an amount insuicient to raise the temperature .of the reaction zone above 800 C., gradually admitting further amounts of nitrogen to the reaction zone to continue the nitriding reaction, and regulating the rate of formation of calcium nitride and hence the rate of generation of exothermic heat by controlling the rate at which said further amounts of nitrogen are admitted and thereby maintain the temperature of the reaction zone between 300 C. and 800 C.

8. In the method of producing calcium nitride;

. further amounts of nitrogen to the reaction zone to continue the nitriding reaction, and regulating the rate of formation of calcium nitride and hence the rate of generation of exothermic heat by controlling the rate at which said further amounts of nitrogen are admitted and thereby maintain the temperature of the reaction zone between 300 C. and 800 C.

9. In the method of producing calcium nitride, the improvement which comprises placing a chargeof calcium in a closed reaction zone,y

evacuating the reaction zone to remove Aair and moisture therefrom, heating the calcium to maintain it at a temperature between 300 C. and 800 C., admitting a limited amount of nitrogen to the reaction zone to initiate the nitriding react-ion and limit the exothermic heat generated by the reaction to an amount insuillcient to raise the temperature of the reaction zone above 800 C., admitting an inert gas to the reaction zone to dilute the nitrogen therein and to maintain the reaction zone under a pressure not less than about atmospheric pressure, gradually admitting further amounts of nitrogen to the reaction zone to continue the nitriding reaction, and regulating the rate of formation of calcium nitride and hence the rate of generation of exothermic heat by controlling the rate at which said further amounts of nitrogen are admitted and thereby maintain the temperature of the reaction zone between 300 C. and 800 C.

PETER P. ALEXANDER.

No references cited. 

